Optimized emission control and maximum fuel burn-up are decisive qualifications for a successful furnace design. Thus, they must especially be taken into consideration in circulating fluidized bed scale-up. A simple proportional scaling up of designs used in smaller systems may easily lead to problems in attempting to provide for a good mixing of fuel, combustion air and fluidized bed solids. Additionally, such designs may suffer from not being capable of providing a uniform furnace temperature within the optimum range and a sufficient heat transfer area. All these problems, which may cause enhanced emissions and less than optimal fuel burn-up, have led to a desire to find alternative solutions. Such solutions have, e.g., included designs with multiple furnaces with a common back pass, providing heat transfer panels and/or partial or full division walls within the furnace, or dividing the lower part of the furnace and the bottom grid with, e.g., a dual wall structure.
Different solutions for sectioning the bottom area of a fluidized bed boiler furnace are known in the prior art. U.S. Pat. No. 4,864,944 discloses a division of a fluidized bed reactor into compartments by partition walls having openings for secondary gas to be distributed in a desired manner into the reactor. The partition walls have ducts which are connected to air supply sources and lead to discharge openings at different heights in the partition walls. Correspondingly, U.S. Pat. No. 4,817,563 discloses a fluidized bed system provided with one or more displacement bodies, which may be provided with lines and inlet openings for introducing secondary gas to segmented sections in the lower reactor.
U.S. Pat. No. 5,370,084 discloses different configurations for effective mixing of fuel in a partitioned circulating fluidized bed boiler, including ducts which feed air into the boiler on the interior walls. U.S. Pat. No. 5,215,042 discloses a CFB reactor divided into compartments by at least one vertical, substantially gas tight partition in the upper part of the combustion chamber. The partition wall comprises cooling tubes and is provided with at least one line with a distributing manifold to feed combustion air into the compartments.
U.S. Pat. No. 4,545,959 discloses a chamber for the treatment of particulate matter in a fluidized bed, comprising a duct with a triangular cross section on the bottom of the chamber, and an arrangement of holes or slots in each of the upwardly sloping side walls of the duct for directing an ancillary gas from the duct into the chamber.
The above-mentioned publications suggest introduction of gas into a reactor chamber, e.g., a furnace chamber, through a partition wall within the chamber. A problem arises, however, as the ducting from the air or gas source chamber to the air or gas injection point may be rather long and cause a high pressure drop. A problem arises also in these conventional supply duct constructions due to solids back sifting, i.e., the problems with solid particles from the furnace tending to flow into the gas supply ducts and an increase in the pressure drop over the gas supply ducts. The increase in pressure drop may be very difficult to attend to or to take into consideration when controlling the gas supply.
Conventional bottom grid nozzle constructions, e.g., those equipped with bubble caps normally reaching upward from the bottom grid, would be exposed to heavy erosion if installed on a vertical partition wall within a fluidized bed, due to very high erosive forces caused by the downward flowing solid particle layers in the vicinity of the wall. In fluidized bed reactor furnaces, solid particles tend to flow upward in the middle of each furnace section and downward along its vertical side walls. Such downward flowing particles come in the lower part of the furnace sections, when the cross-sectional area of the furnace sections abruptly decreases, into intense turbulent motion which may locally lead to very strong erosive forces, e.g., also in the regions of secondary gas inlets. In the prior art no special solution for preventing backsifting into gas nozzles or conduits arranged on partition walls has been disclosed.
It is, therefore, an object of the present invention to provide a fluidized bed reactor with a furnace construction with an improved gas supply configuration.
It is particularly an object of the present invention to provide an improved gas supply configuration suitable for large scale circulating fluidized bed (CFB) boilers.
It is then more specifically an object of the present invention to provide an improved secondary gas supply configuration arranged in a partition wall within the lower part of a boiler furnace.
It is more specifically an object of the present invention to provide a fluidized bed reactor with improved gas supply means, with minimized backsifting of solid particles into gas supply conduits therein.
It is thereby also an object of the present invention to provide a fluidized bed reactor with improved gas supply means with decreased pressure losses in the gas supply means.